The present invention relates to electrochemical devices, in particular to printable, electrochemical transistor devices based on conducting organic materials.
Semiconducting and conducting organic materials, both polymers and molecules, have successfully been included in a large range of electronic devices, e g electrochemical devices, for instance as dynamic colorants in smart windows and in polymer batteries. Reversible doping and de-doping involving mobile ions switches the material between different redox states.
Use has been made of semiconducting polymers for the realisation of field effect transistor (FET) devices. The transistor channel of these devices comprises the semiconducting polymer in question, and their function is based on changes in charge carrier characteristics in the semiconducting polymer, caused by an externally applied electric field. In such transistors, the polymer is used as a traditional semiconductor, in that the electric field merely redistributes charges within the polymer material. One such transistor has been realised, which is adapted for miniaturisation and can be used for the production of integrated circuits consisting entirely of polymer material (PCT publication WO99/10939). A stack of sandwiched layers is described, with either a top-gate or a bottom-gate structure. A transistor device with a similar architecture, also using a polymer as semiconducting material in the channel of the transistor, is described in the European patent application EP1041653.
Another type of transistor device based on organic materials utilises electrochemical redox reactions in the organic material. These devices comprise an electrolyte and a conducting polymer that can be switched between an oxidised and a reduced state. One of these oxidation states then corresponds to low, preferably zero, conductivity in the material, whereas the other oxidation state corresponds to a high conductivity relative to the first state. Electrochemical transistor devices have been used as sensors, e g for detection of oxidant in a solution (see, for review, Baughman and Shacklette, Proceedings of the Sixth Europhysics Industrial Workshop (1990), p 47-61). Furthermore, a transistor of the electrochemical type is reported in Rani et al, J Solid State Electrochem (1998), vol 2, p 99-101. The gate electrode architecture in this prior art transistor is shown in FIG. 1 of this reference.
Problems with electrochemical transistor devices of the prior art include the fact that they are difficult and expensive to manufacture. In particular, no electrochemical transistor devices have been disclosed which are capable of being mass produced. Furthermore, the practical use of prior art electrochemical transistor devices has been hampered by their comparatively high power consumption. Furthermore, materials used in prior art devices suffer from a lack of environmental friendliness, processability and economic production possibilities, There is therefore a need for new and improved electrochemical chemical transistor devices.
One of the objects of the present invention is then to meet this demand, by developing the art of electrochemical transistor devices, and by providing a device with handling, production, disposal and other characteristics superior to those of the prior art.
Another object of the present invention is to provide vide an electrochemical transistor device which can be deposited on a large range of different rigid or flexible substrates by conventional printing methods.
Yet another object of the present invention is to provide an environmentally safe electrochemical transistor device, so that the disposal of the device, along with any support onto which it has been deposited, doesn""t give rise to handling problems, and so that no safety restrictions have to be imposed on the use of the device.
Still another object of the present invention is to make possible new applications of conducting organic materials, using several different properties of such materials in combination.
A further object of the invention is to provide processes for the production of such devices, which processes utilise conventional printing methods or other deposition techniques that are well known, relatively un-expensive and easily scaled up.
The aforementioned objects are met by an electrochemical transistor device as defined in the independent claims. Specific embodiments of the invention are defined in the dependent claims. In addition, the present invention has other advantages and features apparent from the detailed description below.
Thus, a supported or self-supporting electrochemical transistor device is provided, which comprises:
a source contact,
a drain contact,
at least one gate electrode,
an electrochemically active element arranged between, and in direct electrical contact with, the source and drain contacts, which electrochemically active element comprises a transistor channel and is of a material comprising an organic material having the ability of electrochemically altering its conductivity through change of redox state thereof, and
a solidified electrolyte in direct electrical contact with the electrochemically active element and said at least one gate electrode and interposed between them in such a way that electron flow between the electrochemically active element and said gate electrode(s) is prevented,
whereby flow of electrons between source contact and drain contact is controllable by means of a voltage applied to said gate electrode(s).
The architecture of the electrochemical transistor device according to the invention is advantageous in that it makes possible the realisation of a layered transistor device with only a few layers, having for example one patterned layer of material comprising a conducting organic material, which layer comprises source and drain contacts and gate electrode(s), as well as the electrochemically active element. The source and drain contacts and the electrochemically active element are then preferably formed by one continuous piece of said material. The source and drain contacts could alternatively be formed from another electrically conducting material in direct electrical contact with the electrochemically active element, The gate electrode(s) may also be of another electrically conducting material. To provide for the necessary electrochemical reactions, whereby the conductivity in the active element is changed, a solidified electrolyte is arranged so that it is in direct electrical contact with both the active element and the gate electrode(s).
In a preferred embodiment, the source and drain contacts and gate electrode(s), as well as the active element, are all arranged to lie in a common plane, further simplifying production of the device by ordinary printing methods, Thus, the electrochemical device according to this embodiment of the invention uses a lateral device architecture. A layer of solidified electrolyte can advantageously be deposited so that it covers, at least partly, the gate electrode(s) as well as covering the electrochemically active element. This layer of solidified electrolyte may be continuous or interrupted, depending partly on which of two main types of transistor architectures is to be realised (see below).
The electrochemical transistor device according to the invention allows for control of electron flow between source and drain contacts. The conductivity of the transistor channel of the electrochemically active element can be modified, through altering of the redox state of the organic material therein. This is achieved by application of a voltage to the gate electrode(s), which generates an electric field in the electrolyte. In the contact area between electrolyte and electrochemically active element, electrochemical redox reactions take place, which change the conductivity of the organic material. Either the organic material in the transistor channel is modified from a conducting state to a non-conducting state as a result of said redox reactions, or it is modified from a non-conducting to a conducting state.
As is readily appreciated by the skilled person, and in analogy to conventional field effect transistors, the electrochemical transistor device of the invention may readily be made to function as a diode device through short-circuiting of the gate electrode and source contact, or of the gate electrode and drain contact. One non-limiting example of this is described in the description below. However, any configuration of the electrochemical transistor device may naturally be used as a diode in this fashion.
Depending on the precise patterning of the conducting organic material and the electrolyte, the electrochemical transistor device of the invention can either be of a bi-stable or a dynamic type. In the bi-stable transistor embodiment, a voltage applied to the gate electrode(s) leads to a change in conductivity in the transistor channel that is maintained when the external circuit is broken, i e when the applied voltage is removed, The electrochemical reactions induced by the applied voltage can not be reversed, since the electrochemically active element and the gate electrode(s) are not in direct electrical contact with each other, but separated by electrolyte. In this embodiment, the transistor channel can be switched between non-conducting and conducting states using only small, transient gate voltages. The bi-stable transistor can be kept in an induced redox state for days, and, in the most preferred, ideal case, indefinitely.
Thus, the bi-stable transistor embodiment of the present invention offers a memory function, in that it is possible to switch it on or off using only a short voltage pulse applied to the gate electrode. The transistor stays in the conducting or non-conducting redox state even after the applied voltage has been removed. A further advantage with such bi-stable transistors is that close to zero-power operation is made possible, since the short voltage pulses applied to the gate need not be larger than a fraction of the gate voltages needed for operation of a corresponding dynamic device.
In the dynamic transistor embodiment, the change in the redox state of the material is reversed spontaneously upon withdrawal of the gate voltage. This reversal is obtained through the provision of a redox sink volume adjacent to the transistor channel in the electrochemically active element. Also, a second gate electrode is provided, and arranged so that the two gate electrodes are positioned on either side of the electrochemically active element, one closer to the transistor channel, and the other closer to the redox sink volume. Both gate electrodes are separated from the electrochemically active element by electrolyte. Application of a voltage between the two gate electrodes results in the electrochemically active element being polarised, whereby redox reactions take place in which the organic material in the transistor channel is reduced while the organic material in the redox sink volume is oxidised, or vice versa. Since the transistor channel and the redox sink volume are in direct electrical contact with each other, withdrawal of gate voltage leads to a spontaneous reversal of the redox reactions, so that the initial conductivity of the transistor channel is re-established. It is to be stressed that in contrast to electrochemical transistors of the prior art, dynamic transistors according to this embodiment of the present invention revert spontaneously to the initial conductivity state without the need for a reversing bias.
The electrochemical transistor device according to the invention is also particularly advantageous in that it can be easily realised on a support, such as polymer film or paper. Thus, the different components can be deposited on the support by means of conventional printing techniques such as screen printing, offset printing, ink-jet printing and flexographic printing, or coating techniques such as knife coating, doctor blade coating, extrusion coating and curtain coating, such as described in xe2x80x9cModern Coating and Drying Technologyxe2x80x9d (1992), eds E D Cohen and E B Gutoff, VCH Publishers Inc, New York, N.Y., USA. In those embodiments of the invention that utilise a conducting polymer as the organic material (see below for materials specifications), this material can also be deposited through in situ polymerisation by methods such as electropolymerisation, UV-polymerisation, thermal polymerisation and chemical polymerisation. As an alternative to these additive techniques for patterning of the components, it is also possible to use subtractive techniques, such as local destruction of material through chemical or gas etching, by mechanical means such as scratching, scoring, scraping or milling, or by any other subtractive methods known in the art. An aspect of the invention provides such processes for the manufacture of an electrochemical transistor device from the materials specified herein.
However, the invention is not limited to supported devices, as the contacts and electrode(s), electrochemically active element and electrolyte can be arranged in such a way that they support each other. An embodiment of the invention thus provides for a self-supporting device.
According to a preferred embodiment of the invention, the electrochemical transistor device is encapsulated, in part or entirely, for protection of the device. The encapsulation retains any solvent needed for e g the solidified electrolyte to function, and also keeps oxygen from disturbing the electrochemical reactions in the device. Encapsulation can be achieved through liquid phase processes. Thus, a liquid phase polymer or organic monomer can be deposited on the device using methods such as spray-coating, dip-coating or any of the conventional printing techniques listed above. After deposition, the encapsulant can be hardened for example by ultraviolet or infrared irradiation, by solvent evaporation, by cooling or through the use of a two-component system, such as an epoxy glue, where the components are mixed together directly prior to deposition. Alternatively, the encapsulation is achieved through lamination of a solid film onto the electrochemical transistor device. In preferred embodiments of the invention, in which the components of the electrochemical transistor device are arranged on a support, this support can function as the bottom encapsulant. In this case encapsulation is made more convenient in that only the top of the sheet needs to be covered with liquid phase encapsulant or laminated with solid film.
The invention will now be further described with reference to specific embodiments thereof and to specific materials. This detailed description is intended for purposes of exemplification, not for limitation in any way of the scope of the invention as claimed.